Why couldn’t they build escalators within that 25m diameter shaft? Yes it will have to zig zag multiple times down to the platform level (much like escalators in a department store) but surely there would be room for it and much better than elevators only.

I like the idea of a really big 2-level rail tunnel with station incorporated into the tunnel itself.

It mitigates a big concern of mine which was whether we could afford a rail tunnel with multiple CBD stations due to the enormous cost of constructing the underground stations. With this arrangement, station construction is quite straight-forward job.

I was thinking ideal locations for the station entrances (the 25m wide shaft) could be:

- Adelaide (Underground) station: the plaza in front of the Intercontinental (maybe some underground parking might have to be demolished). This could then link to the existing station.

- Hindmarsh Square station: on the southwestern portion of Hindmarsh Square, servicing the eastern part of the CBD. Alternatively as Nort suggested, where the current Rundle St U-Park is located.

Why couldn’t they build escalators within that 25m diameter shaft? Yes it will have to zig zag multiple times down to the platform level (much like escalators in a department store) but surely there would be room for it and much better than elevators only.

Also more simple than excavating a separate shaft for an escalator.

What? How else do you get between the surface and the rail tunnel? The 25m wide tunnel is horizontal at many metres below the surface, you then use small tunnels to connect between that and the surface.

Having said that, cut and cover is still by far the cheapest method, with possibly some small Tunnel-Bored segments in-between where needed. You just need to carefully locate stations along existing roadways or empty (or cheap to demolish) lots. Even in Sydney, they saved a heap of $$ for their NW metro by using smaller tunnels (to accommodate only single-deck trains, not the traditional double-stack). So I wouldn't assume that this method is the panacea of options out there. Having not opened some 17 years later is testament to that.

The higher (or deeper) you go, the more advantage lifts have. Escalators run at a fixed speed, which is slow in Western litigious countries. Lifts can accelerate and decelerate, which gives them an advantage over longer distances.

Having said that, cut and cover is still by far the cheapest method, with possibly some small Tunnel-Bored segments in-between where needed. You just need to carefully locate stations along existing roadways or empty (or cheap to demolish) lots. Even in Sydney, they saved a heap of $$ for their NW metro by using smaller tunnels (to accommodate only single-deck trains, not the traditional double-stack). So I wouldn't assume that this method is the panacea of options out there. Having not opened some 17 years later is testament to that.

Once a TBM is up and running, its costs are small. Cut and cover is cheaper, but if you can't use cut and cover for the whole route, that's when the costs escalate. I can't see how we can have corners in a CBD rail tunnel that are cut and cover, unless the corners are in the parklands.

Smaller tunnels are cheaper, but we have a legacy system with a loading gauge designed for full sized trains. How do the costs of a TBM scale with diameter? If small tunnels are much cheaper, then why stop at metro standards? Build it as a low floor tram with Adelaide's 2.4 m wide loading gauge.